In thin film transistor substrates included in liquid crystal display devices, thin film transistors (hereinafter also referred to as “TFTs”) are used as switching elements of pixels each of which is the smallest unit of an image. Conventionally, TFTs whose semiconductor layers are made of amorphous silicon have been used. However, in recent years, instead of the TFTs including the semiconductor layers made of amorphous silicon, TFTs including oxide semiconductor films as semiconductor layers have been proposed. The TFTs including the oxide semiconductor films as the semiconductor layers exhibit satisfactory characteristics such as high mobility, high reliability, and a low off current, and thus have been actively researched and developed.
A TFT having a bottom gate structure generally includes a gate electrode provided on a glass substrate, a gate insulating film provided to cover the gate electrode, a semiconductor layer provided on the gate insulating film to overlap the gate electrode, and a source electrode included in a source section and a drain electrode included in a drain section which are provided on the gate insulating film to be spaced apart from each other and to overlap the semiconductor layer, wherein a channel section is formed in a portion of the semiconductor layer exposed between the source electrode and the drain electrode. The TFT is covered with an interlayer insulating film provided on the source electrode and the drain electrode. A contact hole extending to the drain electrode is formed in the interlayer insulating film, and an inner surface of the contact hole is covered with a pixel electrode made of a transparent conductive film, thereby electrically connecting the pixel electrode to the drain electrode.
Here, the drain electrode generally has a configuration in which a plurality of metal thin films are stacked. An example of a layered structure of the drain electrode includes a configuration in which a first conductive film made of a titanium film, a second conductive film made of an aluminum film, and a third conductive film made of a molybdenum nitride film are sequentially stacked on the gate insulating film.
In etching to form the contact hole, the contact hole is formed to extend from a surface of the interlayer insulating film to the drain electrode. The etching is performed, for example, by dry etching using a fluorine-based gas as an etching gas. Here, when the contact hole is extended to the drain electrode by the etching gas, and the contact hole passes through the third conductive film, the second conductive film (the aluminum film) is exposed in the contact hole.
When the aluminum film exposed in the contact hole is brought into contact with the etching gas, an aluminum fluoride film is formed on a surface of the aluminum film. Then, oxygen ashing is performed after the dry etching, thereby a surface of the aluminum fluoride film is oxidized, which results in that the surface of the aluminum film is covered with an aluminum oxide film containing fluorine (that is, a passivation coating).
Thus, when an ITO film, or the like is provided as the pixel electrode on the inner surface of the contact hole, the ITO film is brought into contact with the drain electrode, but part of the drain electrode which will be brought into contact with the pixel electrode is covered with the passivation coating, and thus defective conduction is caused, which may lead to quality degradation.
Patent Document 1 discloses that a channel section, a source section, and a drain section of a thin film transistor as well as a pixel electrode, and connection terminal sections at ends of a gate signal line and a source signal line are made of an identical oxide semiconductor film, all layer structures of a thin film transistor array are formed, an opening is formed at a desired location in an uppermost protective insulating film, and a plasma treatment is performed via the opening, thereby simultaneously reducing the resistance of the connection terminal sections, the source section, the drain section, and the pixel electrode to obtain the thin film transistor array.